Evaluation of Bond of Reinforcing Steel in UHPC: Design ......Evaluation of Bond of Reinforcing Steel in UHPC: Design Parameters and Material Property Characterization Jiqiu Yuan,

Evaluation of Bond of Reinforcing Steel in UHPC: Design Parameters and Material Property Characterization

Jiqiu Yuan, Benjamin Graybeal 1

Evaluation of Bond of Reinforcing Steel in UHPC:

Design Parameters and Material Property

Characterization Jiqiu Yuan

1 and Benjamin Graybeal

2

1 Ph.D., P.E., PSI, Inc., Turner-Fairbank Highway Research Center

2 Ph.D., P.E., FHWA, Turner-Fairbank Highway Research Center

6300 Georgetown Pike, McLean, VA, 22101, US

Abstract: As a material on the leading edge of concrete innovation, ultra-high performance

concrete (UHPC) allows the design of innovative structural components and offers the

opportunities to develop new techniques for construction, repair, and retrofit. Compared to the

traditional normal strength concrete which is brittle under tension, UHPC-class materials exhibit

very high compressive strength and enhanced cracking and post-cracking behavior in tension.

The advanced mechanical properties of the UHPC-class material create many opportunities for

innovation in design, but also challenge the use of most current design specifications which are

based on traditional normal strength concrete. This paper presents the evaluation of the bond

between the reinforcing steel and UHPC-class materials. The material properties including the

compressive strength and the tensile characteristics of UHPC are presented. The effect of design

parameters, including the embedment length, concrete cover, and bar spacing on bond strength is

assessed. A total of five different, commercially-available UHPC-class materials with various

fiber contents are included in the evaluation. Results indicate that the embedment length of

reinforcing bar in UHPC can be significantly reduced. Guidance on the embedment of

reinforcing steel in UHPC is provided.

Keywords: UHPC, Reinforcing Bar Bond, Tensile Response, Compressive Strength, Structural Design

First International Interactive Symposium on UHPC – 2016



1. Introduction

As a material on the leading edge of concrete innovation, ultra-high performance concrete

(UHPC) allows the design of innovative structural components and offers the opportunities to

develop new techniques for construction, repair, and retrofit. Among many other advanced

properties of this class material, UHPC-class materials have a very high compressive strength,

low water-to-cementitious materials ratios, and exhibit pseudo stain hardening behavior in

tension due to the high volumetric percentage of fiber reinforcement. The advanced mechanical

properties of the UHPC-class material create many opportunities for innovation in design, but

also challenge the use of most current design specifications which are based on traditional

normal strength concrete. The research presented in this paper is one of the many efforts

conducted at the Turner-Fairbank Highway Research Center of Federal Highway Administration

to characterize this class of material and to facilitate use in structural applications.

The bond between concrete and reinforcing steel is an important topic in structural concrete

as the performance of the structural concrete relies on the composite behavior between

reinforcing steel and concrete. In general, factors that affect the bond strength include the tensile

and compressive strength of the concrete materials, the volume of concrete confining the

reinforcement, which are related to the dimensional parameters like bar embedment length, bar

spacing, and concrete cover, presence of confinement, and reinforcement surface condition.

UHPC materials have a higher tensile strength and greater tensile ductility than conventional

concrete, and the enhanced mechanical behavior of UHPC materials can lead to the reduction, or

even elimination, of the use of steel reinforcement in structural members. This paper presents the

evaluation of the bond between the reinforcing steel and UHPC-class materials. Direct tension

pullout tests were conducted and all above mentioned parameters were investigated. Five UHPC

mixtures were considered in the study. Compressive strength and direct tension tests were

conducted to characterize the UHPC mechanical properties.

2. UHPC Mixtures

Five UHPC mixtures were included in the study. The mix design for each mixture and

corresponding fibers used in each mixture are presented in Table 1 and Table 2, respectively. It

should be noted that the mix designs shown in the table correspond to the mix designs and fiber

contents recommended by the manufacturers. The fiber contents can be adjusted based on the

application, and in this study a fiber content of 2% (by volume) was tested for each UHPC mixture

for comparison purpose. A higher fiber content, either recommended by the manufacturer such as

the 3.1% and 4.6% for UHPC mixes A and C, respectively, or selected by the researchers such as

3.25%, 4%, and 3.25% for UHPC mixes B, D, and E, respectively, were also tested.

3. Testing Methods

3.1. Direct Tension Pullout Test

Direct tension pullout tests, with a novel test specimen design and associated loading apparatus,

were conducted in this study. The test setup was developed to mimic the tension-tension lap

splice configuration that may be encountered in a field-deployed connection system. As shown in

Figure 1, the pullout test specimens were UHPC strips cast on top of precast concrete slabs. The

No.8 (25 mm diameter) bars extended 8 inches (20.3 cm) from the precast concrete slab. UHPC

strips were cast on top of the precast slab with the No. 8 bars in the center of the strips. Each




testing bar was situated so as to be embedded into the UHPC strip and located between two of the

No. 8 bars.

Table 1. UHPC Mix Designs

Designation A B C D E

Mix Design lb/yd3 kg/m

3 lb/yd

3 kg/m

3 lb/yd

3 kg/m

3 lb/yd

3 kg/m

3 lb/yd

3 kg/m

3

Pre-blended

dry powders 3503

† 2078

† 3516 2086 3600 2136 3700 2195 3236 1920

Water 278 165 354 210 268 159 219 130 379 225

Chemical

admixtures 23 13.7 48 28.7 preblended

* 89

†† 53

†† 73 44

Steel fiber

content 416 247 88+179 52+106 613 364 263 156 263 156

Steel fiber

type (refer to

Table 2)

Type I Type II and III Type IV Type V Type V

Steel fiber

content by

volume‡

3.1% 2.0% 4.6% 2.0% 2.0%

†: Not pre-blended but come in as separate ingredient, which include fine silica sand, finely ground quartz flour,

Portland cement, and amorphous micro-silica. *: The chemical admixtures were dry powders and pre-blended with the premix.

††: It includes three chemicals, a modified phosphonate plasticizer, a modified polycarboxylate high-range water-

reducing admixture, and a non-chloride accelerator ‡: Fiber content recommended by the manufacturer, which can be adjusted depending on the application

Table 2. Steel Fiber Properties

Fiber

Type

Fiber

Material

Tensile

Strength Length Cross Section Geometry

Type I Steel 160 ksi

(1100 MPa)

1.18 in.

(30 mm)

Round cross section,

0.022 in. (0.55 mm) diameter

hooked

ends†

Type II Brass coated

steel

≥ 305 ksi

(≥ 2100 MPa)

0.5 in.

(13 mm)


0.012 in. (0.3 mm) diameter straight

Type III Brass coated

steel

≥ 305 ksi

(≥ 2100 MPa)

0.79 in.

(20 mm)



Type IV Brass coated

steel

348 ksi

(2400 MPa)

0.5 in.

(13 mm)



Type V Brass coated

steel

399 ksi

(3750 MPa)

0.5 in.

(13 mm)



†: fibers were adhered together in bundles with a water-soluble adhesive




Note: cso, side cover; 2csi, bar clear spacing to the adjacent No.8 bar; ld, embedment length; ls, lap splice length.

Figure 1. Configuration of Pull-Out Test Specimens.

The pullout test was conducted using the fixture showing in figure 2. A hydraulic jack was

placed on a steel chair, and the steel chair stands on the precast slab. When a pullout force is

applied, the fixture reacts against the precast slab. With such a setup, the reinforcing bars being

tested as well as the extended No. 8 bars are both placed in tension. The UHPC surrounding

these bars transfers the loads between them. This test setup simulates structural configurations

wherein lap spliced reinforcement is loaded in tension.

Figure 2. Pull-Out Test Loading setup.

3.2. Compressive Tests

The compressive tests cylinders were standard 3×6 in. (76.2×152.4 mm) cylinders. The

compressive mechanical testing was completed through modified version of the ASTM C39

Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens. The




employed test method has been used multiple times in the past (Graybeal 2007, Graybeal and

Stone 2012). From the standpoint of the ASTM C39 test method, the load rate was increased from

35 psi/second (0.24 MPa/second) to 150 psi/second (1.0 MPa/second) due to the high compressive

strength of UHPC and the duration of test that would result from the slower load rate.

3.3. Direct Tension Tests

Understanding the tensile response of UHPC is critical to use this type of material. A simple and

reliable direct tension test was developed at the FHWA’s Turner-Fairbank Highway Research

Center (Graybeal and Baby 2013) and was used in this study. The test method was loosely based

on the existing standardized tensile test method for metals described in ASTM E8/E8M (8),

commonly referred to as the “dog bone test”. An illustration of the specimen shape and gripping

setup is shown in Figure 3. Prismatic specimens with dimension of 2×2×17 in. (51×51×432 mm)

are used and tapered aluminum plates are glued on two opposite faces at each end of the

specimen using a high strength, high-stiffness epoxy. The specimen is axial loaded with a

hydraulic-actuated, computer-controlled load frame. The load and deformation within the gage

length are recorded. Further details can be obtained from Graybeal and Baby (2013).

Figure 3. Direct tension test method setup. (from Graybeal and Baby 2013)

The test method allows for capturing the tensile response of the UHPC materials, especially

for the pseudo strain hardening behavior. Graybeal and Baby (2013) summarized four idealized

phases of the tensile response of UHPC materials based on direct tension test, shown in Figure 4.

The phases include an elastic phase until the occurrence of the first cracking, the multi-cracking

phase where the matrix cracks repeatedly and the fibers bridging the cracks are engaged, the

crack straining phase characterized by the crack saturation and the increase of the crack openings

in the existing cracks, and the final localization phase where all the deformation is dominated by

the widening of an individual crack and the behavior is defined by debonding and pullout fiber

mechanisms.




Figure 4. Idealized uniaxial tensile mechanical response of UHPC. (from Graybeal and Baby

2013).

4. Results

4.1. Bond Strength

A typical bar stress versus slip curve for a steel bar embedded in UHPC is presented in Figure 5.

The bar stress fs is calculated as the applied load divided by the cross section area of the bar. The

displacement is measured along a loaded portion of the bar at about two inches (51 mm) above

the top surface of UHPC strip. For comparison, the bar stress versus slip curve for a traditional

concrete [compressive strength of 4700 psi (32.5MPa)] with the same setup as UHPC is also

included in Figure 5. It should be noted that after the bar stress reaches the maximum at bond

failure, the bar stress dropped quickly for the bar embedded in concrete while it gradually

decreased for the bars in UHPC.

Figure 5. Bar Stress versus Slip at Loaded Bar End.

Over 500 reinforcing bar bond tests were conducted. The factors that affect reinforcement

bond strength, including the embedment length, bar spacing, concrete cover, bar size, bar type,

0.0 1.3 2.5 3.8

0

138

276

414

552

690

828

966

0

20

40

60

80

100

120

140

0 0.05 0.1 0.15

Slip (mm)

Ba

r S

tres

s f s

(MP

a)

Ba

r S

tres

s f s

(ksi

)

Slip (in.)

Example UHPC Response

Example Conventional

Concrete Response

Splitting failure with

instant stress drop




and concrete compressive strength were extensively evaluated. The main findings can be

summarized below. Further details can be found in research by Yuan and Graybeal (2014) and

Yuan and Graybeal (2015).

Increasing the embedment length of the reinforcing bar increases bond strength.

The relationship between the bond strength and the bonded length for bar embedded in

UHPC is nearly linear, indicating that UHPC exhibits enhanced performance as

compared with traditional high-strength concrete.

Bond strength increases as the clear cover increases.

Non-contact lap splice specimens exhibit higher bond strength than contact lap splice

specimens, due to the fact that the tight spacing in contact lap splice limits the ability of the

fiber reinforcement to locally enhance the mechanical resistance of the UHPC.

When the bar clear spacing is so large that the induced diagonal cracks from the pullout

force will not intersect with the adjacent bars, the adjacent bar will not help stop the

propagation of the diagonal cracks and the bond strength becomes a function of the

tensile mechanical properties of the UHPC.

Models that used bar spacing and side cover to predict reinforcing bar bond strength in

conventional concrete may need to be reevaluated in consideration of the added crack

propagation resistance provided by the fiber reinforcement in UHPC.

An increase in the compressive strength of the UHPC results in an increased bond strength.

For bars with larger diameter, the bond strength decreases.

Bars that yield before bond failure have less ultimate bond strength than similarly

configured high-strength bars that do not yield before bond failure.

The epoxy-coated bars have lower bond strength than similarly configured uncoated bars.

As part of the large test matrix evaluated in the study, the bond tests with different UHPC

mixtures are presented in this paper and the results are summarized in Table 3. All of these

specimens were tested with No. 5 bars and were detailed to have an embedment length of 8db, a

side cover of 3db, and a bar center to center spacing of 4 in. (102 mm) (clear spacing between 2db

and ls), where db is the diameter of the steel bar. As shown, all the UHPC mixtures with a 2% (by

volume) fiber content and compressive strength between 13.3 and 16.3 ksi (92 and 112 MPa)

reached a bar stress at bond failure well above 75 ksi (517 MPa). When higher fiber contents

were used, UHPC mixtures A, B, and D had similar bond strength as the corresponding mixtures

with 2% (by volume) fiber, while UHPC mixtures C and E exhibited improved bond strength.

Table 3. Bar stress at bond failure for different UHPC formulas

UHPC A UHPC B UHPC C UHPC D UHPC E

Fiber content, by volume, % 2.0 3.1† 2.0

† 3.25% 2.0% 4.6

† 2.0

† 4.0 2.0

† 3.25

Compressive Strength, ksi 16.0 15.2 16.3 14.7 13.6 10.8 13.5 14.5 13.3 14.6

fs, max at bond failure, ksi

avg. 123 124 137 124 126 147 132 130 109 136

min. 118 121 127 115 118 131 117 124 100 122

max 126 128 145 142 135 157 143 142 116 145

No. of Tests 3 3 4 5 4 6 10 5 5 5 †:

Fiber content recommended by the manufacturer, which can be adjusted depending on the application.

1 ksi = 6.895 MPa




Design recommendations for steel reinforcing bars embedded in UHPC were developed for

deformed bars reaching the lesser of the bar yield strength or 75 ksi (517 MPa) at bond failure.

These recommendations are presented in Design and Construction of Field-Cast UHPC

Connections (Graybeal 2014) and are reiterated here:

Bar size from No. 4 to No. 11,

Uncoated or epoxy coated bar,

Minimum embedment length of 8db,

Minimum clear cover of 3db,

Bar clear spacing between 2db and ls,

Minimum UHPC compressive strength of 13.5 ksi (93 MPa), and

A maximum flow of 10 in. (254 mm) after 20 drops following the ASTM C1437 tests to

minimize fiber segregation.

4.2. Compressive Strength

The compressive strength for the five UHPC mixtures at ages between one day and 125 days are

reported in Figure 6. Each of the UHPC mixtures were tested with two different fiber contents.

All the cylinders were air cured in a laboratory condition with an ambient temperature of 72 °F

(22 °C). The observations based on the compression strength test are as follows.

The tested UHPC materials gained strength quickly (laboratory condition with air

curing). UHPC D, which used an accelerator, had the most rapid strength gain reaching a

compressive strength around 13.5 ksi (93 MPa) at one day after casting. Other UHPC

materials gained strength slightly slower than UHPC D and could reach a compressive

strength higher than 13.5 ksi (93 MPa) within 4 or 5 days.

Similar to conventional concrete, most strength development happened well within the

first 28 days and the strength gain after that is minimal.

Increasing the fiber content does not seem to have much effect on the compressive

strength. The mixtures with higher fiber content had slightly higher, if not the same,

compressive strength than the corresponding mixes with 2% (by volume) fiber.

Figure 6. Compressive Strength.

34

69

103

138

172

5

10

15

20

25

0 14 28 42 56 70 84 98 112 126 140

Co

mp

ress

ive

Str

eng

th,

MP

a

Co

mp

ress

ive

Str

eng

th,

ksi

Age, days

UHPC A 2.0% UHPC A 3.1%

UHPC B 2.0% UHPC B 3.25%

UHPC C 2.0% UHPC C 4.6%

UHPC D 2.0% UHPC D 4.0%

UHPC E 2.0% UHPC E 3.25%




4.3. Direct Tension Test

The tensile performance of the UHPC mixtures is presented in Figure 7. All the direct tension

specimens were tested at the same age when the pull-out bond test was conducted. The

compressive strength of the UHPC mixtures can be found in Table 3. As shown in Figure 7(a),

all UHPC mixtures (except for UHPC C) with 2% (by volume) fiber exhibited similar ductile

tensile behavior, where the specimens first reached the first cracking strength (end of the elastic

phase, refer to Figure 4), then the stress sustained or increased with more defamation, followed

by stress decrease due to crack localization. For UHPC C, the axial tensile stress started to

slowly decrease shortly after reaching the first cracking strength. All UHPC mixtures in Figure

7(a) had the first cracking strength higher than or equal to 0.8 ksi (5.5 MPa). When higher fiber

content was used, the UHPC mixtures exhibited different tensile behaviors. Two obvious

changes comparing the mixtures in Figure 7(b) with the mixtures the Figure 7(a) are higher first

cracking strength and more obvious cracking straining phase (refer to Figure 4). More analysis is

being performed and will be reported in the future publications.

Figure 7. Tensile stress-strain response for (a) UHPC mixtures with 2% (by volume) fiber,

and (b) UHPC mixtures with more than 2% (by volume) fiber.

5. Conclusions

The research discussed herein focused on assessing the bond strength of deformed reinforcing

bar in UHPC and UHPC material properties in terms of compressive and tensile responses. A

total of five commercial-available UHPC mixtures were evaluated. It was found that the bond

behavior of deformed reinforcing steel in UHPC is different from that in traditional concrete in

many aspects and the reinforcing steel development length in UHPC can be significantly

reduced. Guidance on the embedment of deformed reinforcing bars into UHPC is provided.

Regarding mechanical properties, all the tested UHPC materials reached a compressive strength

higher than 13.5 ksi (93 MPa) within 5 days (laboratory condition with air curing). While the

fiber content does not significantly effect on compressive strength, it does significantly affect the

tensile response, with sustained tensile capacities ranging from 0.8 ksi (5.5 MPa) to more than

1.2 ksi (8.3 MPa).

0.0

2.8

5.5

8.3

11.0

13.8

0.0

0.4

0.8

1.2

1.6

2.0

0.000 0.002 0.004 0.006 0.008 0.010

Av

era

ge

Ax

ial

Str

ess

(MP

a)

Av

era

ge

Ax

ial

Str

ess

(ksi

)

Average Axial Strain

UHPC D 2.0%

UHPC B 2.0%

UHPC E 2.0%

UHPC A 2.0%

UHPC C 2.0%0.0

2.8

5.5

8.3

11.0

13.8

0.0

0.4

0.8

1.2

1.6

2.0

0.000 0.002 0.004 0.006 0.008 0.010

Av

era

ge

Ax

ial

Str

ess

(MP

a)

Av

era

ge

Ax

ial

Str

ess

(ksi

)

Average Axial Strain

UHPC D 4.0%

UHPC B 3.25%

UHPC A 3.1%

UHPC E 3.25%

UHPC C 4.6%




6. Acknowledgments

The research was funded by the U.S. Federal Highway Administration. This support is gratefully

acknowledged. This research project could not have been completed were it not for the dedicated

support of the federal and contract staff associated with the FHWA Structural Concrete Research

Program. PSI, Inc. provided laboratory support to FHWA under contract DTFH61-10-D-00017

through the duration of this research project. The publication of this report does not necessarily

indicate approval or endorsement of the findings, opinions, conclusions, or recommendations

either inferred or specifically expressed herein by the Federal Highway Administration or the

United States Government.

7. References

ASTM C39-11, “Standard Test Method for Compressive Strength of Cylindrical Concrete

Specimens,” ASTM Book of Standards Volume 04.02, ASTM International, West

Conshohocken, PA, 2011.

Graybeal, B., “Compressive Behavior of Ultra-High Performance Fiber-Reinforced Concrete,”

ACI Materials Journal, V. 104, No. 2, Mar.-Apr. 2007, pp. 146-152.

Graybeal, B., and B. Stone, “Compression Response of a Rapid-Strengthening Ultra-High

Performance Concrete Formulation,” Federal Highway Administration, National Technical

Information Service Accession No. PB2012-112545, September 2012.

Graybeal B., Baby F., “Development of Direct Tension Test Method for Ultra-High-Performance

Fiber-Reinforced Concrete”, ACI Materials Journal. V. 110, No.2, 2013, pp. 177-186.

Graybeal, B., “Design and Construction of Field-Cast UHPC Connections,” FHWA-HRT-14-

084, U.S. Department of Transportation, Federal Highway Administration, 2014.

Yuan, J and Graybeal, B., “Bond Behavior of Reinforcing Steel in Ultra-High Performance

Concrete,” FHWA-HRT-14-090, U.S. Department of Transportation, Federal Highway

Administration, 2014.

Yuan, J. and Graybeal, B., "Bond of Reinforcement in Ultra-High Performance Concrete,” ACI

Structural Journal, V.112, No. 06, 2015, pp. 851-860.


Submission ID: 102

Title: EVALUATION OF BOND OF REINFORCING STEEL IN UHPC: DESIGN PARAMETERS AND MATERIAL PROPERTY CHARACTERIZATION

Status: Accept

Reviewer 1:

Author Comments: General Comments:

The abstract leads the reader to believe that the correlation between bond strength and

material properties such as UHPC compressive and tensile strength will be discussed, but

this is not presented in the paper. The content of the abstract or the content of the paper

should be revised. Response: Updated. Thanks.

The frequently use the phrase “2 volumetric % fiber”. This wording is cumbersome, appears

odd to the reader, and should be revised. “2% fiber by volume” is more straight forward. Response: Updated.

Specific Comments:

1.Abstract - “commercial-available” should be “commercially-available”.

Response: Updated. Thanks.

2.Introduction, second paragraph, first sentence – This sentence is cumbersome and should

be re-written for clarity.


3.Introduction, second paragraph, second sentence – The word “confining” should be

changed to “surrounding” or something similar. The use of this word is misleading.

Response: The “confining” indicates that the surrounding concrete restricts the free

movement of steel. The authors believe it is an appropriate description here.

4.Section 3.1, first paragraph, forth line – The “f” in figure need to be capitalized.


5.Section 3.2, fourth line – The word “engaged” should be changed to “used”


6.Section 4.1. eighth bullet point – “large diameter” bars should be more well defined here.

Is “large” No. 10 and greater? Define.

Response: The largest bar tested in the study is No.11. It is stated in later discussion where

design guidance is provided. Here, as a general discussion for the effect of the bar size, it is

true in general (which is also true in conventional concrete). The authors do not think it is

necessary to add the actual bar size here.

7.Section 4.1, General – There is no mention of reinforcing bar type / grade when the

authors discuss results from the different UHPC types.

Response: Other UHPC mixtures were evaluated with a certain bar type and size (Grade 120

No. 5 bar) and the results were compared among different UHPCs. The effect of bar

type/grade was only evaluated with Type D UHPC and it is believed that they will have the

same effect when other UHPCs were tested.


8.Section 4.1, design recommendation – At no point in the paper do the authors define the

terms “db” or “ls”. For completeness these should be properly defined.

Response: added on page 7 where the term db was first introduced.

9.Section 4.2, second bullet – “convention” should be “conventional”

Response: changed. Thanks.

10.Section, 4.3 forth sentence – “ductal” should be “ductile”

Response: changed. Thanks.

Reviewer 2:

Author Comments: As the length of contribution is limited, I miss some more details of

test results, for instance bond test results. But I understand that 500 test result would fill up one contribution. Nice and comprehensive text full of information. Leave it as is.

Response: Thanks.


Documents

Evaluation of Bond of Reinforcing Steel in UHPC: Design ......Evaluation of Bond of Reinforcing Steel in UHPC: Design Parameters and Material Property Characterization Jiqiu Yuan,